To produce vertical trenches that can have a large depth, typically at least 15 metres, it is known to use drilling machines usually called excavators, themselves being of well-known type. Such drilling rigs are described in particular in European Patent Application No. 1 231 327 filed in the name of the Applicant. These rigs essentially consist of a frame suspended from the lower end of a pulley block. The frame of the excavator is equipped at its lower end with two pairs of excavator drums each driven by a motor, preferably a hydraulic motor. Depending on the nature of the ground to be dug, excavating tools of different types are fastened to the shell of the excavator drum.
When it is desired to produce the trench in a hard or very hard soil, the excavating tools often consist of rollers or cutting wheels mounted so as to rotate freely on the periphery of the shell of the drum. These rollers are equipped with studs which enable the hard soil to be perforated and thus the drilling to be carried out. However, mounting the excavating tools on the shell of the drum in this case raises difficulties.
These difficulties will emerge more clearly with reference to FIG. 1, appended hereto. Represented in this figure is the central core 12 of the excavator that constitutes the lower end of the excavator frame. Mounted on this core are two excavator drums 14 and 16 that are represented by their trench in FIG. 1. Each drum, or more accurately each drum shell 14a, 16a, has a certain width l along the direction of the rotation X-X′ axis of the drums. To produce the vertical trench, it is necessary that the excavating tools mounted on the shells enable drilling over a width L at least equal to the width l of the shells. However, when it is required to drill into a hard or very hard soil, mounting the rollers or cutting wheels constituting the excavating tools raises considerable difficulties on account of the very large forces that have to be absorbed by the rollers and therefore in particular by the bearings and the axles on which these rollers are rotationally mounted.
To attempt to solve this problem, U.S. Pat. No. 5,924,222 has described excavator drums which consist of cylindrical shells on which are mounted frustoconical rollers that are angularly offset in relation to the shells and have, towards the external faces of the shell, a rotation axis inclined sufficiently to enable the axles of the rollers to be mounted on two end bearings. This enables the very large mechanical forces applied to the axles and to the bearings to be absorbed. By virtue of the high degree of inclination of these axles, it is necessary that the rollers have a high degree of conicity.
However, this solution, which consists in using highly frustoconical rollers steeply inclined with respect to the mid-plane of the shells, does not enable a high efficiency of the excavating tools to be obtained.
One object of the present invention is to provide an excavator drum which effectively enables the trench to be excavated over the total width of the shells of the excavator drums while at the same time making it possible to obtain a much greater efficiency of the tool than is obtained by the technique described above.